Many different types of filter have been demonstrated. These include various types of absorption filtering, dielectric single or multilayer filtering, interferometric filtering (Fabry-Perot, Michelson, Mach-Zehnder, Sagnac etc) and grating filtering.
A conventional (prior art) Mach-Zehnder filter consists of a beam splitter (or coupler) which splits the input light into two paths and a beam combiner (or coupler) to combine the light again. If the two paths have different lengths, the Mach-Zehnder filter has a wavelength dependent transmission characteristic.
The split-beam Fourier filter (SBFF) described in this document is a special type of filter which can give the same characteristic transmission as a Mach-Zehnder filter as well as more complex characteristics by simply splitting the beam of a fibre beam expander with an appropriate transparent element or elements so that different parts of the beam travel different optical path lengths. This gives highly stable performance and is ideally suited to incorporation in a single mode fibre beam expander.